A signal oscillator used for a sender generally tends to experience more deterioration over time as the cumulative time taken for data sending becomes longer; and finally becomes unable to send out a signal with a desired intensity, and reaches its life span. Thus, in order to suppress such deterioration over time, it is disclosed that the operating currents of a plurality of signal oscillators are monitored and the running of the plurality of signal oscillators is leveled on the basis of the monitoring result, and thereby deterioration over time due to a specific signal oscillator being intensively used is suppressed (for example, see Patent Literature 1).